Image sensors are generally semiconductor devices which can convert optical images into electrical signals. Such image sensors can typically be classified as either charge coupled device (CCD) image sensors or complementary metal oxide silicon (CMOS) image sensor (CIS). CMOS image sensors include photodiodes and MOS transistors in unit pixels that detect electrical signals in the respective unit pixels in a switching mode, thereby realizing an image. Some CMOS image sensors may have a structure that includes a photodiode portion, to convert light signals into electrical signals, and a transistor portion, to handle the electrical signals, which are horizontally arranged. In other words, horizontal CMOS image sensors have a structure in which photodiodes and transistors lie horizontally adjacent to each other on a substrate. Accordingly, horizontal CMOS image sensors include an additional region where photodiodes may be formed, which decreases a fill factor region and limits resolution.